This invention relates in general to the art of providing protective and decorative coatings of the type most typically applied in industry by spray painting techniques. More specifically, this invention relates to a flexible and stretchable sheet material that can be bonded to various substrates, including exterior automotive panels, to achieve desired protective and decorative effects and thereby reduce or eliminate the need to utilize spray painting processes in the manufacturing operation. The invention also relates to a process for manufacturing such sheet material on an industrial scale.
To facilitate an understanding of the many advantages of the invention and to provide for ease of description, the invention will be specifically described hereinafter with reference to providing protective and decorative coatings on exterior automotive panels, but it should be understood that the automobile is only one of many different types of products in whose manufacture the invention can be beneficially applied.
As pointed out in an article by Alan J. Backhouse entitled xe2x80x9cRoutes To Low Pollution Glamour Metallic Auomotive Finishesxe2x80x9d, Journal of Coatings Technology, Vol. 54, No. 693, pages 83-90, October 1982, there is a growing need to reduce the amount of atmospheric pollution caused by solvents emitted during industrial painting processes. Many different approaches to meeting this need have been proposed. For example, efforts have been made to replace the solvent-based paints typically used for automobiles with water-based paints. As another alternative, work has been done to facilitate the use of high solids formulations which will result in proportionately less emission of organic solvents. However, the application of automotive finishes is a highly demanding art because of the extremely high quality of the surface finish required and because of the very common application of metallic finishes to provide what Backhouse refers to as xe2x80x9chigh stylistic effectsxe2x80x9d. Accordingly, past efforts to replace the low viscosity, low-solids-content paint formulations conventionally used in spray painting operations in the automotive industry have met with only very limited success.
A much more promising approach to solving the problem is to eliminate entirely the need for spray painting operations to provide the necessary protective and decorative coating on exterior automotive panels. Elimination of spray painting, or a substantial reduction in the extent of its use, would not only be environmentally beneficial in reducing atmospheric pollution, but would be extremely beneficial from a cost savings standpoint in that spray painting operations are wasteful of the paint to such an extent that more than half of the paint may be lost as waste material. A means for achieving such goal exists through the use of a pre-formed thermoplastic sheet material which can be glued or otherwise securely bonded to the panel to provide the protective and decorative coating. Such techniques are well known and widely used in industry, and have been utilized for such purposes as interior automobile panels as described, for example, in U.S. Pat. No. 3,551,232 issued Dec. 29, 1970. This art is summarized in the aforesaid patent in the following words:
xe2x80x9cIt is present day conventional practice to make structural members consisting of a relatively rigid substrate to which is bonded as a surface or cover layer a synthetic resin sheet. The surface layer may be smooth or embossed and may be suitably colored to provide a desired decorative effect. The substrate may be formed of a relatively rigid synthetic resin, such as polystyrene, or sheet metal, and the surface layer and the substrate are bonded together to form a laminate. Such structures may be used for a variety of purposes such as interior auto-mobile panels, glove compartment doors, and the like.
A convenient and economical method for making such articles involves the application of a suitable adhesive to the surface of the substrate and then vacuum forming the decorative cover layer over the adhesive layer of the substratexe2x80x9d.
The objective of U.S. Pat. No. 3,551,232 is to overcome the problems of bubbling and blistering of the resin sheet that tend to occur in the vacuum-forming process. It achieves this by use of an adhesive containing an inert particulate filler which minimizes the entrapment of air.
To employ a process of the type described in U.S. Pat. No. 3,551,232 with exterior automotive panels presents a much greater challenge. The surface appearance of such panels is of critical importance, so that it is necessary not only to avoid such problems as bubbling or blistering caused by entrapped air, but to provide a protective and decorative coating that will equal or exceed in many respects, the quality of a spray-painted surface and that is equally capable of providing the xe2x80x9chigh stylistic effectsxe2x80x9d that are of growing importance in the automotive marketplace. Furthermore, exterior automotive panels present a particular problem in view of the great difficulty of smoothly adhering a flexible sheet material to a substrate which may possess complex curvature and the even greater difficulty of doing so while maintaining over the entire surface of the panel a uniform degree of the color intensity exhibited by the coating.
Heretofore, efforts have been made to produce a flexible and stretchable sheet material having these capabilities through the application of one or more paint layers to the surface of a thermoformable polymeric support. However, such efforts have, prior to this invention, been unsuccessful in that the sheet material has suffered from various defects which have rendered its commercial use impractical. Thus, for example, prior efforts have resulted in products which exhibit numerous coating defects and in which the paint layer does not have the high degree of uniformity which permits the sheet material to undergo the severe stresses of the thermoforming process and yet provide a product which meets the exacting quality standards of an exterior automotive finish.
It is toward the objective of providing a flexible and stretchable sheet material which is capable of being stretched to conform to a three-dimensional substrate, such as an exterior automotive panel, to provide a high quality defect-free protective and decorative coating of uniformly attractive appearance, that the present invention is directed.
In accordance with this invention, it has been found, most unexpectedly, that a flexible and stretchable sheet material that is (1) capable of meeting the demanding requirements of an exterior automotive finish and (2) capable of withstanding the stretching and bending forces involved in the process of bonding it to exterior automotive panels, can be produced through exacting control of the thickness and thickness uniformity of the support combined with the use of precision coating techniques to provide exacting control of the thickness and thickness uniformity of the coatings. Moreover, as hereinafter described, such coating techniques provide essentially defect-free coatings having a continuous substantially uniform quality and appearance.
The sheet material of this invention comprises a thin flexible support sheet and a protective and decorative layer adhered to one surface of the support sheet. The support has heat-softening and tensile elongation properties which adapt it to use in the thermoforming process and the protective and decorative layer has heat softening and tensile elongation properties that are compatible with those of the support so as to permit symmetrical elongation, whereby the sheet material can undergo substantial elongation by the application of stretching forces without crazing of the protective and decorative layer nor delamination of the protective and decorative layer from the support. The sheet material has a substantially unstressed relaxed state and a relaxed area and is heat softenable to a substantially plastic state in which it is plastically expandable and formable over irregular surfaces to an extended state having an extended area up to at least 50% greater than the relaxed area. The protective and decorative layer has a substantially uniform quality and appearance both in the relaxed and extended states of the sheet material. The sheet material can be stretched to conform to a three-dimensional substrate and bonded thereto to achieve a smooth and wrinkle-free protective and decorative coating of uniformly attractive appearance.
The invention also provides a process for the manufacture of the sheet material comprising the steps of: providing a thin, flexible support sheet; providing a fluid protective and decorative film-forming composition; forming a laminar flow of the composition and directing the flow into contact with the surface of the support to form thereon a protective and decorative layer of substantially uniform thickness; and converting the protective and decorative coating to a dry and tack-free state. The protective and decorative film-forming composition is advantageously coated on the support having thereon an adhesion-promoting tie-layer. The process of this invention provides an essentially defect-free coating with a high degree of uniformity of layer thickness and a substantially uniform quality and appearance.
In a very important embodiment of the invention, the protective and decorative layer is a paint layer comprising a film-forming binder, a colorant and generally flat light-reflecting particles which are effectively oriented by the laminar flow process of this invention, that is, they are preferentially oriented substantially parallel to the surface of the coating. This provides a layer characterized by a high degree of geometric metamerism.
In another important embodiment of the invention, the sheet material includes a topcoat comprising a crosslinked polymer and the support, paint layer and topcoat layer have heat-softening and tensile elongation properties which render the sheet material suitable for use in a thermoforming process and which are mutually compatible so as to permit symmetrical elongation, whereby the sheet material can undergo substantial elongation by the application of stretching forces without crazing or delamination of the paint and topcoat layers, and the support, paint and topcoat layers are of sufficiently uniform character that the sheet material exhibits substantially the same uniformity of appearance before and after the thermoforming process.
In another embodiment of this invention, there is provided a structural element comprising a three-dimensional substrate having securely bonded to at least one surface thereof in a uniform wrinkle-free state a protective and decorative coating formed from the flexible and stretchable sheet material described above.
In a further embodiment of the invention, a method of applying a protective and decorative coating on a three-dimensional substrate includes the steps of heating and stretching the above-described sheet material as it is brought into conforming association with at least one surface of the substrate and securely bonding it to the substrate to form thereon a smooth and wrinkle-free protective and decorative coating.
In yet another embodiment of the invention, there is provided a method of molding a shaped article and simultaneously providing such article with a protective and decorative coating formed from the above-described sheet material on at least one surface thereof.
As explained hereinafter, the sheet material of this invention is adapted for use in a thermoforming process in which it is elongated and is especially advantageous in providing essentially defect-free coatings of continuous substantially uniform quality and appearance both in the relaxed and extended states of the sheet material.
Other advantageous features will become readily apparent upon reference to the following description of the preferred embodiments when read in light of the accompanying drawings.